1. Field
This disclosure relates to a light-emitting device, and particularly, to a white light-emitting device.
2. Description of the Related Art
In general, a white light from a lamp should be designed to have a high color rendering index (“CRI”) as well as high luminous efficiency. However, it is known that there is a trade-off between high luminous efficiency and high color rendering index. For example, the luminous efficiency may be increased by emitting single color light of about 555 nanometers (nm), but the color rendering index may be increased by emitting light having a wide spectrum evenly covering a visible light region.
On the other hand, red is important in terms of red/green contrast or color reproducibility, but R9, a saturated red color rendering index, is not included in calculating a general color rendering index or a R1-R8 average color rendering index (in general referred to as Ra) but is regarded as a special color rendering index. In addition, not much attention has been paid to a red light emitter due to low luminous efficiency. Since a white light emitting diode (“LED”) fabricated by applying a yttrium aluminum garnet (“YAG”) phosphor having a wavelength of 550 nm has a low R9 for red, an attempt has been made to apply a red phosphor thereto to improve the color rendering index. However, such a phosphor has a wide light emitting spectrum including a wavelength region that is invisible to human eyes, and thus may have decreased efficiency.